Refill

Well, it's officially time for a CO2 refill. The tank is definitely falling away from 950-1000 psi, so that probably means the liquid CO2 has been consumed. Because I'm literally running on fumes at this point, the working pressure has rised to 50-60 psi with no adjustment to the coarse valve. So I've been closing up the needle valve not only to keep the bubble count from getting too high, but to bring it down due to all the trimming I've been doing.

Oh, you're wondering what running on fumes has to do with a higher working pressure? I did too. This diagram from Docsmachine.com helps to understand what's going on. Normally, the CO2 tank has a constant pressure as the liquid evaporates to whatever the vapor-pressure-related equilibrium is (in the 950-1000 psi range depending on its temperature). When the pressure drops because the liquid has completely evaporated, the pressure drops from that level, and now the spring has an easier time pressing on the reservoir CO2. That releases gas that raises the working pressure until a new equilibrium is achieved. As the tank pressure continues to drop, the working pressure will continue to rise and the bubble output will follow suit. Therefore, I am attacking the problem by reducing the bubble count below 1 per second.

In order to wait twice as long until my next refill, I'd like to actually use the solenoid on a timer so that I can stop dosing CO2 at night without killing my tank inhabitants in the morning. To play it safe for now, I'm manually plugging in and unplugging the solenoid myself to prevent a discharge. I'm still not quite sure why it takes to long for the bubbling to respond to binary solenoid changes, but I'm guessing it has something to do with internal system friction and the finite time it takes for pressure to equilibrate.

In order to help me in the future and do this EI dosing thing correctly, I'd like to really be able to nail down my CO2 levels at all times, specifically, holding it steady at 30 ppm. I've read about a really neat way to do it using something called a "drop checker". Here are do-it-yourself building instructions as well as instructions for how to fill it. We'll see if I can find it at any stores tomorrow while I shop for bioballs and interesting/useful driftwood/rocks (if I actually make it to any).